We have recently discovered a human cancer cell surface protease termed seprin that may be a key integral membrane protein involved in basement membrane proteolysis by cancer cells. Tumor cell lines selected for higher cell surface expression of seprin have an increased ability to degrade the extracellular matrix at membrane sites of invasion. Breast cancer seprin is immunologically related to melanoma and placental seprin, for which cDNA probes are currently available, and Northern blot analysis shows that seprin mRNA is expressed in both the invasive MDA-MB-231 and non-invasive MCF-7 breast cancer cell lines. Cross reacting monoclonal antibodies demonstrate that seprin is expressed on the surface of MDA-MB-231 and MCF-7 cells, but localizes to surface protrusions and lamellipodia only in the invasive MDA-MB-231 cells. A seprin-associated molecule first identified in placenta, plO5, bears amino acid sequence homology to dipeptidyl peptidase IV. A putative anti-plO5 monoclonal antibody (C27) labels the plasma membrane and ruffles of MDA-MB-231 cells but demonstrates a polarized localization in the non-invasive MCF-7 cells and is not expressed in several other non-invasive breast cancer cell lines. This project focuses on determining the role of seprin and seprin-associated plO5 in the progression of breast cancer cells to malignancy. Thus, based on currently available monoclonal antibodies and cDNA probes, as well as breast cancer specific probes to be obtained in this study, we will test the hypothesis that functional cell surface seprin may be an important modulator of breast carcinoma cell invasion and thus an excellent target for strategies in prognostication and therapy. We plan to 1) purify breast carcinoma seprin using existing cross reacting antibodies, and develop new monoclonal antibodies that inhibit seprin activity; 2) identify the role of seprin and C27 antigen in the progression or breast cancer cells to malignancy; and 3) determine the requirement for translocation of membrane proteases to invasive cellular protrusions during localized degradation of the extracellular matrix.